Comparisons With Other EruptionsThe May 18, 1980, eruption of Mount St. Helens was exceeded in "size" by many other eruptions, both in historic times and in the recent geologic past.

For the study of earthquakes, two standard measures of the "size" of the seismic event are commonly used: the Richter Magnitude Scale (based on energy released as measured by seismometers) and the Modified Mercalli Intensity Scale (based on damage caused as assessed by people). Although some attempts have been made to develop a scale to compare the relative sizes of volcanic eruptions, none has yet been adopted for general use. Volcanologists have proposed and used various schemes to rank eruptions, and these generally included one or more of the following factors - height of eruption column, volume of material erupted, distance and height of hurled blocks and fragments, amount of aerosols injected into the upper atmosphere, and duration of eruption. All these factors relate directly or indirectly to the total amount of energy released during the eruption. Perhaps the two most commonly used and directly measurable factors are eruption volume and height of the eruption column.

The May 18 eruption ejected about 0.3 cubic mile of uncompacted ash, not counting an unknown but probably much smaller amount that was deposited in the atmosphere or too diffuse to form measurable deposits. This volume of ash is less than those of several earlier eruptions of Mount St. Helens and considerably less than the ejecta volumes of some historic eruptions elsewhere. The 1815 eruption of Tambora (Sumbawa, Indonesia) ejected about 30 to 80 times more ash than did Mount St. Helens in 1980. The 1815 Tambora eruption ranks as the largest known explosive eruption in historic times. But even the Tambora eruption pales by comparison with the gigantic pyroclastic eruptions from volcanic systems such as Long Valley Caldera (California), Valles Caldera (New Mexico), and Yellowstone Caldera (Wyoming)--which, within about the last million years, produced ejecta volumes as much as 100 times greater.

Ejecta volume, in cubic miles

Some scientists recently proposed the Volcanic Explosivity Index (VEI) to attempt to standardize the assignment of the size of an explosive eruption, using ejecta volume as well as the other criteria mentioned earlier. The VEI scale ranges from 0 to 8. A VEI of 0 denotes a nonexplosive eruption, regardless of volume of erupted products. Eruptions designated a VEI of 5 or higher are considered "very large" explosive events, which occur worldwide only on an average of about once every 2 decades. The May 1980 eruption of Mount St. Helens rated a VEI of 5, but just barely; its lateral blast was powerful, but its output of magma was rather small. The VEI has been determined for more than 5,000 eruptions in the last 10,000 years. None of these eruptions rates the maximum VEI of 8. For example, the eruption of Vesuvius Volcano in A.D. 79, which destroyed Pompeii and Herculaneum, only rates a VEI of 5. Since A.D. 1500, only 21 eruptions with VEI 5 or greater have occurred: one VEI 7 (the 1815 Tambora eruption), four of VEI 6 (including Krakatau in 1883), and sixteen of VEI 5 (counting Mount St. Helens in 1989 and El Chichon, Mexico in 1982). Considered barely "very large," the eruption of Mount St. Helens in May 1980 was smaller than most other "very large" eruptions within the past 10,000 years and much smaller than the enormous caldera-forming eruptions - which would rate VEl's of 8--that took place earlier than 10,000 years ago.

The number of casualties and extent of destruction also have been used to compare the "bigness" of volcanic eruptions. For obvious reasons, such comparisons are limited at best and misleading at worst. Some of the most destructive eruptions have not been in other terms "very large." For example, mudflows triggered by the November 1985 eruption of Nevado del Ruiz (Colombia) killed more than 25,000 people - resulting in the worst volcanic disaster in the 20th century since the catastrophe at Mont Pelee in 1902. Yet, the eruption was very small, producing only about 3 percent of the volume of ash ejected during the May 1980 eruption of Mount St. Helens. As the table below clearly shows, of the seven greatest volcanic disasters in terms of casualties since A.D. 1500, only two of them (Tambora and Krakatau) qualify as "very large" eruptions (VEl's greater than 5) in terms of their explosive force.

The May 1980 eruption of Mount St. Helens has a higher VEI (5) than five of the deadliest eruptions in the history of mankind, but it resulted in the loss of far fewer lives (57). Loss of life would have been much greater if a hazard warning had not been issued and a zone of restricted access had not been established.

Subsequent Eruptive Activity

Since May 18, 1980, Mount St. Helens has remained intermittently active, and through early 1990 and at least 21 more periods of eruptive activity had occurred renewing again in October 2004 and continuing through mid-2008. Geologists view these periods of activity as eruptive episodes of one eruption that continued through the decade, rather than separate eruptions. The first of these smaller but significant eruptive episodes began early Sunday morning, May 25, 1980, when Mount St. Helens explosively erupted ash and formed an eruption column that rose to a maximum altitude of 9 miles. At least one pyroclastic flow accompanied the vertical ash emission. Although this eruption was considerably less energetic and voluminous than that of May 18, it nonetheless caused much concern because of memories of the events of the previous Sunday. Variable winds dispersed ash over southwestern Washington and neighboring Oregon, producing small to moderate ash falls in communities that had been spared the ash fall of May 18.

For the next 2 weeks, Mount St. Helens remained relatively quiet, puffing gas but little ash. Meanwhile, rootless steam-blast eruptions continued in the northern periphery of the apron of the pyroclastic flows in the valley of the North Fork of the Toutle River. On clear nights, aerial observers reported seeing glows in the vent within the crater, interpreted to reflect the near-surface presence of very hot rock or magma, although no lava was extruded. On June 12, the volcano again erupted, generating ash falls to the south-southwest and pyroclastic flows down its north flank. The June 12 episode was similar to that on May 25 in style and volume, and both eruptions were preceded by volcanic tremor a few hours in advance.

Probably within hours following the explosive activity on June 12, but hidden by poor visibility, very stiff magma began to ascend in the vent, slowly oozed onto the crater floor, and formed a bulbous lava dome (a mound of sticky lava) about 1,200 feet in diameter and 150 feet high. Such lava domes commonly form at composite volcanoes following major explosive eruptions. The formation what was to be the first of three domes at Mount St. Helens during the 1980s was confirmed by observers on June 15, when visibility over the volcano improved.

Mount St. Helens erupted again in three explosive pulses during the afternoon and evening of July 22. The July eruptive episode was preceded by several days of measurable expansion of the summit area, heightened earthquake activity, and changed emission rates of sulfur dioxide and carbon dioxide. Plumes of ash rose to altitudes of between 6 and 11 miles. The July 22 events destroyed most of the dome formed in mid-June, and pyroclastic flows poured through the north breach of the summit crater and overrode earlier flows of May and June. No dome developed after the end of the explosive activity, which ejected only about one-tenth as much ash as did the May 25 and June 12 explosions.

During the next 3 months, explosive episodes occurred on August 7 and on October 16-18. These events were preceded by differing combinations of the following precursors: increased earthquake activity, volcanic tremor, changed rates of gas emission, and expansion of the crater near the vent. Both episodes produced ash-and-gas clouds and pyroclastic flows. A small dome formed after the August explosions but was blasted away at the start of the October activity.

Another dome began to form within 30 minutes after the final explosion on October 18, and within a few days it was about 900 feet wide and 130 feet high. The October 16-18 eruptive episode turned out to be the last major explosive activity at Mount St. Helens during the 1980s. All subsequent eruptive episodes, beginning with the December 27, 1980-January 3, 1981 episode, involved predominantly nonexplosive, dome-building activity that added material to the October dome. During the remainder of 1981, five such dome-building episodes, accompanied by little or no ash ejection, took place: February 5-7, April 10-12, June 18-19, September 6-11, and October 30-November 2.

Three eruptive episodes occurred in 1982: March 19-April 9, May 14-18, and August 18-23. A moderate explosion initiated the March-April episode, ejecting ash 9 miles high and melting snow in the crater that generated a mudflow which eventually entered the North Fork of the Toutle River. Dome growth followed this explosion. The May and August episodes reverted to the nonexplosive style and involved only dome growth. For about a month following the end of the May event, however, small explosions were frequent and at times impressive, producing spectacular vertical plumes of gas and rock debris many thousands of feet high. These were visible from Portland, Oregon (50 miles away) and, on occasion, even from Seattle (100 miles away). More than 500 similar small explosions occurred sporadically until 1986, during times when the dome was not growing. Scientists believe that these small explosions were caused either or both of the following processes: steam-blasts triggered when cold infiltrating rain and snow melt came in contact with the subsurface hot part of the dome and magma conduit, and rapid expansion of gas carried by the magma itself. Many of the explosions occurred in the late spring and early summer, when snow melt is at a maximum.

First lava dome [81 K]

A year-long episode of eruptive activity began on February 7, 1983. It was preceded by several explosions on February 2-4 that resembled those just described. The largest of the February explosions produced plumes of gas and ash 2 to 4 miles high. These explosions ripped open a gash high on the east flank of the dome, through which lava extruded several days later. The February dome-building activity culminated in the formation of a spine-like protrusion of lava that rose about 100 feet above the summit of the dome. This spine, as did other smaller ones, lasted for only about 2 weeks and then collapsed into a heap of rubble. Slow growth of the dome took place more or less continuously throughout 1983, accompanied at times by small explosions from a crater at the crest of the dome. During this time, the dome grew not only as lava was added to its surface (extrusion) but also as magma entered and inflated the dome as if a water balloon were being filled (intrusion).

A spine-like protrusion of lava rose about 100 feet above the summit of the dome during February 1983 (Photograph by Thomas Casadevall). ( A similar but much larger lava spine rose in 2005-2007 which was referred to as the "whale's back". )

The year-long eruptive episode ended in February 1984, but activity resumed on March 29 and lasted until about April 2, producing a small lava lobe on the surface of the dome. More small explosions occurred during the spring and summer months, and on September 10-12 a large lava lobe was extruded, accompanied by major distention of the north part of the dome at unprecedented rates that approached 120 feet per day!

Dramatic dome growth during the 1981-85 period: (Top) Viewed in August 1981 from a camera station at a distance of about 0.5 mile; the dome is about 535 feet high, or nearly as tall as the George Washington Monument. (Bottom) Seen in August 1985 from the same station; the dome has grown considerably wider and another 220 feet higher (Photographs by Lyn Topinka).

Considerable distention also occurred during the next eruptive episode, May 24-June 10, 1985, when the southern third of the dome was pushed more than 300 feet southward by an intrusion, leaving a deep gorge in its wake that stretched across the dome like a "smile" on a golf ball. Only a little lava oozed from the floor of this gorge (called a graben by geologists), but more than 5 million cubic yards of magma entered the dome and remained stored there.

The last two significant eruptive episodes of the 1980s occurred on May 8-13 and October 21-24, 1986. Each episode was similar, producing a large lava lobe and major internal expansion of the dome. Small explosions preceded the activity in May but not in October. These two episodes, as well as those in September 1984 and May 1985, were accompanied by much more intense earthquake activity than was associated with prior dome-building episodes. This change in style of precursory seismicity, together with the changed style of eruptive activity - from mainly extrusion to about equal occurrence of extrusion and intrusion - suggested that the magma had become stiffer and less able to rise easily to the surface than previously. Moreover, the rate of sulfur dioxide release had progressively decreased with time, suggesting that the magma was "running out of gas." Thus, scientists were not surprised that no dome growth took place during the last 3 years of the 1980s. Perhaps the series of eruptive episodes that began in 1980 has ended. However, in late 1989 periods of increased, though still weak, seismicity occurred, and in early December 1989 and early January 1990, at least three very small explosions deposited thin layers of ash in the crater. Perhaps these events either represent a "dying gasp" or are forerunners of continued or heightened eruptive activity. Time will tell - and it has. It turned out the dome growth remained stagnant until October 2006 when a new eruptive phase began and a second dome began growing beside the first.

Scientists making measurements inside Mount St. Helens' crater to monitor the growth of the "composite dome" (Photograph by Lyn Topinka).

The dome at Mount St. Helens is termed a composite dome by scientists, because it represents the net result of many eruptive events, not just one event. The dome-building process may be pictured as the periodic squeezing of an upward-pointing tube of toothpaste or caulking compound. The process is dynamic, involving the upward movement of new material, cracking and pushing aside of old material, sloughing of material from steep surfaces of the dome, and occasional small but violent explosions that blast out pieces of the dome. These processes result in earthquakes and measurable changes in shape of the dome and nearby crater floor; study of the earthquakes and changes in shape enables prediction of the onset of eruptive episodes.

At the start of 1990, the composite dome was about 3,480 feet by 2,820 feet in diameter and rose about 1,150 feet above the low point on the adjacent crater floor. It has a volume of about 97 million cubic yards, less than 3 percent of the volume of the volcano (about 3.5 billion cubic yards) removed during the landslide and lateral blast on May 18, 1980. If the dome resumes growth at its average rate of the 1980s (about 17 million cubic yards per year), it would take nearly a century to fill in the summit crater and more than 200 years to rebuild Mount St. Helens to its pre-1980 size.

Possible Future Behavior

For a few intensively monitored volcanoes, scientists in recent years have greatly improved their capability to predict when and sometimes even where an eruption might take place, with lead times on the order of several days or less. For example, the current ability to predict eruptive episodes at Mount St. Helens represents a major advance; since 1980, all episodes (except for one very small event in 1984) have been successfully predicted several days to 3 weeks in advance. Even for accurately predicted eruptions, however, there is no way to anticipate their size or duration. Moreover, scientists are not yet able to forecast accurately the long-term future behavior of volcanoes. For example, scientists cannot answer with any certainty the following questions about Mount St. Helens: Is the intermittent activity of the 1980s over? Will another large explosive eruption comparable to that of May 18, 1980, take place within the next decade or even century? Will lava flows accompany future eruptions?

Most earth-science studies are concerned with past events, and the axiom that "the present is the key to the past" is fundamental to these studies. In recent years, as earth scientists have been asked repeatedly to look forward in time, the axiom that "the past and present are keys to the future" has become increasingly significant. Clues to the possible future behavior of Mount St. Helens are gleaned from its past eruptive history. During the past 50,000 years, Mount St. Helens has experienced nine "eruptive periods," not counting the activity of the 1980s. The term eruptive period is informally used by geo-scientists for a segment of a volcano's eruptive history encompassing a series of eruptive episodes closely associated in time and/or type of eruptive processes or products; such periods are separated by dormant intervals, generally of longer duration.

The most recent and best known of the pre-1980 eruptive periods began with a major explosive eruption in 1800 A.D. For the next 57 years, this event was followed by intermittent relatively small explosive eruptions, lava flows, and the extrusion of a lava dome. Assuming that Mount St. Helens behaves as it did in the 19th century, the present activity could continue intermittently for years, possibly decades. Such activity could include the outpouring of lava flows (not observed to date), as well as renewed dome growth and small-to-moderate explosive events. The chance of another catastrophic landslide and blast comparable to that of May 18, 1980, is exceedingly low. The past history of the volcano suggests, however, that one or more explosive eruptions with heavy ash fall comparable to that of the May 18, 1980, eruption might occur before Mount St. Helens returns to a dormant state. This history of intermittent activity is one of the most important reasons why scientists continue to monitor the volcano to detect the intensive, sustained seismic activity and ground deformation that can be expected to accompany any massive infusion of new magma required to break the pattern of dome building in the 1980s and to feed an explosive eruption of major proportions.

View of Mount St. Helens from Johnston Ridge on March 30, 1987 (Photograph by Lyn Topinka); through April 1990, the composite dome inside the crater had not enlarged since the last eruptive period in October 1986 until a new dome began growing in October of 2004.

Bezymianny Volcano, Kamchatka, U.S.S.R., erupted violently in March 1956 and has been intermittently active since then. This photograph, taken in 1977, shows the growing composite dome of Bezymianny rising above the crater rim. Mount St. Helens could similarly remain active for several decades, and continuing dome growth ultimately may fill the crater to form a new conical summit (Photograph courtesy of the Kamchatka Volcano Station).

Continuing Volcanic and Hydrologic Hazards

The continuing intermittent eruptive activity at Mount St. Helens poses volcanic and hydrologic hazards for the foreseeable future, especially if eruption frequency and vigor increase. Specific hazards - ash fall, pyroclastic flows, mudflows, and floods-were described by scientists years before they became stark realities on May 18, 1980. Since then, as the volcano settled into a pattern of episodic, moderate and generally nonexplosive activity, the severity and regional impact of ash fall, lateral blasts, and pyroclastic flows have diminished. Given Mount St. Helens' alternations between explosive and nonexplosive activity in its past, however, the possibility of violent eruptions and attendant hazards in the future should not be discounted.

Considerable hazards still exist in the immediate vicinity of the volcano's present summit -the amphitheater-like crater, with its episodically active and growing composite lava dome. Whenever the composite dome enlarges, chances increase for collapses of its steep, irregular sides. Such collapses, in turn, could hurl rock fragments onto the crater floor and possibly trigger small pyroclastic flows through the crater breach and down the north flank of the mountain toward Spirit Lake. Rockfalls from the unstable steep walls of the crater have been common since the formation of the huge crater, posing a local but significant hazard to scientists working within it. Scientists and other people working close to or within the volcano's crater - within the "restricted zone" established by the USFS - must remember these hazards and take safety precautions.

As an example of continuing hazards, mudflows triggered by the eruption of March 1982 poured down the north flank of Mount St. Helens and reached the valley of the North Fork Toutle River (Photograph by Thomas Casadevall).

Lava flows from Mount St. Helens pose little direct hazard to people or property because such flows are likely to be sluggish and, therefore, should not move fast or far from the vent. Anyone in good health should be able to out walk or outrun the flows, and no major civil works are near enough to the volcano to be overrun by lava flows. However, such flows can melt snow and ice and thus could cause minor debris flows, mudflows, and floods.

Given the current, relatively quiet, eruptive behavior of Mount St. Helens, debris flows and floods at present constitute the greatest hazards related to volcanic activity. The potential for mudflows and floods was increased by the existence of new ponds and lakes formed when the debris avalanche of May 1980 blocked parts of the preexisting drainage to serve as natural dams. As these natural dams are composed of loose, easily erodible volcanic debris, they are structurally weak and could fail, which would trigger mudflows and floods.

Scene at the shore of Spirit Lake showing the Army Corps of Engineers' project to control the rise of the lake level. A pump barge is at the upper left (Photograph by Lyn Topinka).

Devastating mudflows or floods or both could be triggered by any or all of the following: heavy rainfall during storms, melting of snow and ice by hot eruptive products (especially pyroclastic flows), or sudden failure of one of the lakes impounded by the debris avalanche deposits. During winter - the time of peak precipitation and maximum snowpack- the risks of mudflows and floods increase significantly. Normal precipitation in the Mt. St. Helens area is heavy, especially on the volcano's upper slopes, where the average annual rainfall totals 140 inches. In a normal winter, the snowpack on the volcano's higher slopes can be about 16 feet thick. Thus, scientists and civil authorities were rightly concerned about the high potential for mud flows and floods, and the Army Corps of Engineers began to take engineering measures-including sediment-retention structures and channel dredging in the drainages most vulnerable to mudflow and flood hazards.

The inlet of the diversionary tunnel at Spirit Lake, part of the permanent system now used to regulate lake level. The tunnel is 11 feet in diameter and 1.5 miles long. Large logs and other debris deposited by the lateral blast on May 18, 1980, float on the lake surface around the inlet (foreground). (Photograph by Lyn Topinka in October 1986).

As an example of the flood hazards in the Mount St. Helens region, in August 1980 the failure of a natural debris dam caused the rapid draining of a 250-acre-feet lake in the Toutle River Valley near Elk Rock. (One "acre-foot" of water is equal to the volume contained in a one-foot layer covering one acre , or about 325 thousand gallons.) The ensuing flood damaged a partially constructed sediment retention structure and heavy channel-maintenance equipment in the North Fork of the Toutle River. Fortunately, no injuries or deaths resulted. During the next 9 months, no large floods happened, largely because no high-intensity rainfalls occurred even though the total precipitation for the winter and spring of 1980-1981 was near normal. There were no major mudflows or floods the following winter-spring, again because rainfall generally was low intensity. Meanwhile, the levels of the lakes impounded by natural dams, however, gradually rose due to rainfall and runoff.

By the fall of 1982, the debris dams for three of the largest lakes-at Spirit Lake, Coldwater Creek, and South Fork Castle Creek-were becoming substantially filled, thereby increasing the risk of catastrophic flooding should the dams fail or be over-topped. The Corps of Engineers, which in 1981 started construction of controlled outlets at Coldwater and Castle Lakes, began also to control the rise of the level of Spirit Lake by an interim plan of barge-based pumping and discharge into outlet channels. The USGS and the National Weather Service installed flood-warning systems in the Toutle and Cowlitz River Valleys. By March 1983, Spirit Lake contained 360,000 acre-feet of water, the lake at Coldwater Creek had 67,000 acre-feet, and that at South Fork Castle Creek had 1·9,000 acre-feet. Scientists and engineers estimated that a breach of the natural dam at South Fork Castle Creek, the smallest of the three lakes, could unleash mudflows and floods comparable to those triggered by the May 18, 1980, eruption of Mount St. Helens. The Corps of Engineers and other Federal, State, and county agencies initiated a variety of projects to mitigate the growing hydrologic hazards. These mitigation projects required many people and much equipment to work in the hazardous zones close to the volcano. To ensure the safety of the mitigation operations, scientists had to intensify their monitoring efforts not only of the volcano itself but also of the debris-clogged drainage systems.

Though less severe now than in the early 1980s, mudflow and flooding hazards should exist for many years, until such time as the slopes and areas around Mount St. Helens, by re-vegetation and normal erosion, return to or approach their pre-eruption forest cover, stream gradients, rates of flow, discharge, and channel dimensions. As part of a long-term plan to cope with the continuing hydrologic hazards, the Corps of Engineers, in April 1985, completed the construction of a 1.5 mile-long diversionary tunnel at Spirit Lake. This permanent tunnel system replaced the temporary, barge-based pumping operations to regulate the lake's water level.

Since May 1980, the natural recovery of the drainage system around Mount St. Helens has been substantial. Yet, during this recovery period, some roads in the region sustained significant damage from mudflows and floods, and a number of homes were lost because of stream-bank erosion. However, much more damage would have occurred if it were not for the construction of sediment-retention structures, dredging, and other engineering mitigation measures taken by the Army Corps of Engineers. It should be emphasized, however, the recovering drainage system has not been subjected to a truly major storm during the past decade. Thus, scientists, engineers, and government officials must continue to closely assess and monitor the continuing volcanic and hydrologic hazards. Human efforts to control the floods and sedimentation are designed not only to gain time to lessen the impact of hydrologic hazards until the natural "healing" of the drainage systems around Mount St. Helens is complete, but also to try to guide, if possible, the healing process.

Aerial view of a sediment-retention structure, in the North Fork of the Toutle River, used to reduce mudflows and flood hazards (Photograph courtesy of the Army Corps of Engineers). Mount St. Helens can be seen on the skyline (upper left).

Scientists' ChaIlenge and Opportunity

The eruptive activity of Mount St. Helens has provided a good test for scientists who faced the challenge of obtaining, relaying, and explaining in easily understandable terms the information needed by the Federal, State, and local officials charged with land management and public safety. It should be reemphasized, however, that a quick response at Mount St. Helens was possible only because decades of systematic research before 1980 had contributed to a good understanding of the volcano's eruptive behavior and potential hazards. Additionally, the Mount St. Helens activity also has provided scientists a unique opportunity to learn much about the dynamics of an active composite volcano. The results of studies completed and in progress have improved the understanding of eruptive mechanisms and should refine a forecasting capability not only for Mount St. Helens but also for similar volcanoes in the United States and elsewhere.

Scientists of the David A. Johnston Cascades Volcano Observatory (CVO) collecting gas samples in Mount St. Helens as part of the geochemical monitoring program (Photograph by Kathy Cashman).

When the 4.2-magnitude earthquake occurred on March 20, 1980, seismologists of the University of Washington and the USGS began a round-the-clock effort to expand the monitoring and to evaluate the seismic activity. As the number of earthquakes in creased over the next few days, USGS and other scientists discussed with officials of the Gifford Pinchot National Forest the significance of the seismic activity, the safety of USFS facilities near the volcano, and the need to close its upper slopes because of snow avalanche and other hazards. USGS scientist Donal Mullineaux arrived on the scene the evening of March 25, and an emergency coordination center was set up at the USFS headquarters in Vancouver. The next day, Mullineaux, one of the foremost experts on Mount St. Helens described the possible types of eruptions and associated volcanic hazards at a meeting of representatives from government and industry. Following the meeting, the USFS, State, and county officials decided to extend the area of closure beyond the immediate flanks of the volcano. The same day (March 26), the general nature of potential eruptive activity and volcanic hazards was discussed again at a joint USFS-USGS press conference. An official announcement of a Hazard Watch for Mount St. Helens was issued by the USGS at 8 a.m. PST on March 27. By 12:36 p.m. that day, the first eruption of Mount St. Helens in over a century had begun.

By the time the eruptive activity was into its second week, 25 to 30 scientists were on hand carrying out a wide variety of monitoring and volcanic-hazard-assessment studies. These scientists participated in daily meetings and briefings with USFS and other officials and provided advice on the locations of hazardous zones for use, such as the selection of sites for roadblocks to control access around the volcano. All decisions regarding access and restricted areas, however, were the sole responsibility of the USFS, State of Washington, and other land managers for the Mount St. Helens region. Ironically, in 1980 the section of land containing the summit crater was owned by the Burlington Northern Railroad; it has since been acquired by USFS by land exchanges. On March 31, an on-site, comprehensive, volcanic-hazards assessment was presented at another meeting of agencies responsible for public safety. On April 1, a large-scale volcanic-hazards map was prepared for use by these agencies. A news release was issued by the USGS on what might be expected should the activity develop into a "major eruptive phase." Scientists contributed geo-technical and volcanic hazards information essential for preparing the "Mount St. Helens Contingency Plan" issued by the USFS on April 9. Although the possibility that the collapse of the rapidly deforming "bulge" on the north flank could trigger a magmatic eruption was considered and discussed with officials at various meetings in late April, scientists could not be sure that such an event would actually occur, let alone estimate its timing or size.

The early recognition of the potential hazards of the bulge on Mount St. Helens' north slope and the systematic measurement of its extremely rapid growth led scientists to advise the USFS that hazards were increasing. Accordingly, the USFS, State, and county officials enforced closure zones. Had these access-control measures not been taken, the catastrophic events of May 18 would have resulted in considerably more human deaths and injury. An element of luck also saved many lives. The catastrophe began hours before the scheduled departure of a caravan of landowners permitted by officials to enter the controlled access area to inspect their properties and cabins. Also, had the eruption occurred on any other day than Sunday, many more people authorized to enter the restricted areas (such as loggers, USFS personnel, and government officials) would have been at work and exposed to the danger.

Legislation passed by Congress in 1974 made the Geological Survey the lead Federal agency responsible for providing reliable and timely warnings of volcanic hazards to State and local authorities. Under this mandate, and recognizing the need to maintain systematic surveillance of Mount St. Helens' continuing activity, the USGS established a permanent regional office at Vancouver, Washington, after the May 18, 1980, eruption. On May 18, 1982, the office at Vancouver was formally designated the David A. Johnston Cascades Volcano Observatory (CVO), in memory of the Survey volcanologist killed 2 years earlier. Staffed by about 90 permanent and part-time employees-geologists, geophysicists, hydrologists, geo-chemists, technicians, and supporting personnel-the CVO not only maintains a close watch on Mount St. Helens but also serves as the headquarters for monitoring other volcanoes of the Cascade Range in Washington, Oregon, and northern California. In recent years, the CVO staff has also participated in studies of eruptions or unrest at other volcanoes in the western United States and elsewhere in the world. The Cascades Volcano Observatory is a sister observatory to two other volcano observatories operated by the USGS: the Hawaiian Volcano Observatory, founded in 1912, has pioneered or refined most of the modern volcano-monitoring methods used in the world today; and the Alaska Volcano Observatory, established jointly by the USGS and the state of Alaska in 1988, studies the volcanoes of the Alaskan Peninsula and Aleutian Islands.

Throughout the 1980s, the ability of scientists at CVO and the University of Washington to provide warnings for dome-building eruptive episodes has been exceptional. Indeed, for all episodes (except for one small event) since May 1980, scientists using data from seismic, ground deformation, and volcanic gas monitoring-have provided reliable forecasts from several hours to several days, even weeks, in advance of these events. The table (p. 50) gives a typical example of the timely information for one 1982 eruption given to government officials charged with emergency management and to the general public via news releases.

Sketch map showing the close-in monitoring network at Mount St. Helens. The seismic network, jointly operated by the USGS and the University of Washington, covers an area much larger than that shown in the diagram--encompassing the entire State of Washington. EDM stands for electronic distance-monitoring.

At Mount St. Helens, the track record for predicting eruptions, especially dome-building ones, is better than any previously accomplished for any volcano in the world. Our improving predictive ability, however, has not been tested by any large explosive eruptions - that is until October 2004 when the USGS did predict the renewed eruptive activity at Mount St. Helens by a few days.

Mount St. Helens has provided, and will continue to provide, an unprecedented opportunity for scientific research on volcanism. Relatively easy accessibility and a dense network of monitoring instruments have made Mount St. Helens a natural laboratory at which scientists can study processes typical of volcanoes elsewhere along the circum-Pacific "Ring of Fire." As Mount St. Helens is monitored continuously before, during, and after each eruptive episode, and its eruptive products are regularly sampled for chemical and other laboratory analyses, the information being compiled and interpreted yields a better understanding of Mount St. Helens in particular, and other composite volcanoes in general. Moreover, the monitoring techniques now being used at Mount St. Helens and other Cascade volcanoes are the same as, or variations of, those used to monitor the active Hawaiian volcanoes. Thus, in the rather young science of volcanology, there is a rare opportunity to compare the effectiveness of these techniques on two contrasting kinds of volcanoes--the Hawaiian shield volcanoes, which typically erupt non-explosively, and the Cascade composite volcanoes, which typically erupt explosively. Scientists have learned that data from all types of monitoring are helpful regardless of the type of volcano. From such comparative studies, they will be able to determine which techniques are the most effective and reliable for monitoring each type of volcano. With such tools and broadened knowledge, scientists may be entering a new epoch in volcanology, in which significant advances in understanding volcanic phenomena will be achieved, accompanied by a sharpened ability to forecast and mitigate volcanic hazards.

Mount St. Helens National Volcanic Monument

Despite the troubled economy in early 1980s, tens of thousands of visitors flocked to the area surrounding Mount St. Helens to marvel at the effects of the eruption. On August 27, 1982, President Reagan signed into law a measure setting aside 110,000 acres around the volcano as the Mount St. Helens National Volcanic Monument, the nation's first such monument managed by the USFS. At dedication ceremonies on May 18, 1983, Max Peterson, head of the USFS, said,"we can take pride in having preserved the unique episode of natural history for future generations." Since then, many trails, viewpoints, information stations, campgrounds, and picnic areas have been established to accommodate the increasing number of visitors each year. Beginning in the summer of 1983, visitors have been able to drive to Windy Ridge, only 4 miles northeast of the crater. From this spectacular vantage point overlooking Spirit Lake, people see firsthand not only the awesome evidence of a volcano's destruction, but also the remarkable, gradual recovery of the land as re-vegetation proceeds and wildlife returns.

Mountain climbing to the summit of the volcano has been allowed since 1986, and winter exploration of the crater itself is a difficult but rewarding adventure. A majestic Visitor Center was completed in December 1986 at Silver Lake, about 30 miles west of Mount St. Helens and a few miles east of Interstate Highway 5; by the end of 1989, the Center had hosted more than 1.5 million visitors. Scheduled for opening in 1992 or 1993 is an interpretation complex in the Coldwater Lake-Johnston Ridge area, from which visitors will be able to view the inside of the crater and its dome from the site of David Johnston's camp on the morning of May 18, 1980.

The National Volcanic Monument preserves some of the best examples and sites affected by volcanic events for scientific studies, education, and recreation. Intensive monitoring of the volcano is now all the more important to ensure the safety of the scientists and the monument's visitors.

People view exhibits about Mount St. Helens at the U.S. Forest Service Visitor Center (Photograph by Jim Quiring, USFS Mount St. Helens Volcanic National Monument).

This small tree, protected by a snowbank, survived the devastation in the lateral-blast zone and remains among the vegetation beginning to grow on the scarred land (Photograph by Peter Lipman).

View of Mount St. Helens from the north in April 1981, with Spirit Lake in the middle ground (Photograph by Lyn Topinka).

View of Mount St. Helens from the north in August 1984 (Photograph by Lyn Topinka). The fireweed was among the first plant life to reappear after the devastation on May 18, 1980.

Source: Most of this historical information is from a general interest publication prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment written by Bob Tilling who was Chief of the Office of Geochemistry and Geophysics, at USGS' headquarters in Reston, Virginia from 1976-81 and was in charge of the USGS studies before, during, and after the 18 May 1980 catastrophic eruption of Mount St. Helens.

ground-deformation measure- ment as a small explosion takes place. This photograph shows the CVO scientist making a ground-deformation measure- ment as a small explosion takes place. This photograph shows the laser beam transmitter positioned atop a heavy 12-foot-high steel tower centered over a benchmark used as the mea surement reference point. Such towers allow measure ments to be made at Mount St.Helens during the winter despite deep snow (Photograph by Lyn Topinka).